How to fix this seam. Video included

you shouldn't have to, painting over seams with pass through and clone stamp is once of the core selling points, I use it all the time.  I had a look at the video and couldn't see anything obvious I would have to look at the project / mesh etc to figure out what's going on or it's just guessing.  

You could just tri-planer that texture instead of uv project and scale it then just paint some variation over the top but the clone stamp should work.  If you still see seams with tri-planer then I suspect you need to make sure your UV islands are all scaled correctly i.e. they are all the same scale / texel density for the bits you want to pait over so double check that with a uv check pattern as well.  If they are wrong as long as you don't move the mesh around you can just re-unrap it and re-import the mesh and let substance re-project all the strokes and re-bake any mesh maps (as long as you haven't painted loads just in UV space) make back up first before trying that 🙂

Mark